Collective nature of light emission by atomic ensembles yields fascinating effects such as
superradiance and radiation trapping even at the single-photon level. Photon emission is
influenced by virtual transitions which lead to collective frequency shift (Lamb shift). For large
samples light emission is also affected by time retardation due to the finite value of the speed
of light. In my talk I will review results on collective photon emission obtained in our group
during last 10 years. In particular, I will discuss how virtual and nonlocal effects modify
dynamics of the atomic system yielding peculiar temporal features in the state evolution and
present recent findings for superradiant and trapped states of two dimensional atomic shells. I
will also show that quantum mechanical evolution equations describing single-photon emission
(absorption) by atomic ensembles can be written in a form equivalent to the semiclassical
Maxwell-Bloch equations which substantially simplifies treatment of the problem. Finally I will
discuss some experiments on superradiance and radiation trapping.